Non-invasive, low-intensity pulsed ultrasound has been shown to be effective for transdermal treatment of fresh fractures (38% reduction in clinical and radiographic healing time) and fracture nonunions, while less effective for large scale segmental defects. The mechanism through which LIPUS acts is poorly understood, which has weakened enthusiasm for widespread clinical use. Although the exact mechanism is not known, in vitro cell studies have shown that osteoblasts respond to LIPUS exposure much the same as they respond to various forms of mechanical loading, suggesting that LIPUS may impart a physical, acoustic radiation force on cells to stimulate a response. Scaffold-based tissue engineering has also been proposed for the repair of fractures but, unlike LIPUS, large-scale fracture non-unions and has been used clinically to a limited extent. Each approach has its merits but to date have not been combined in a synergistic way; that is, LIPUS has not been applied to implanted deformable hydrogels for bone defect repair. The goal of this proposal is to combine LIPUS generated acoustic radiation force with hydrogel-based tissue engineering with the belief that both approaches together will enhance repair over either approach alone. Using LIPUS-generated force capable of imparting physical forces on cells, it is our intention to design hydrogel scaffolds that are 1) able to deliver encapsulated viable cells in vivo, 2) be physically deflected by LIPUS generated acoustic radiation force after implantation and during the healing process and 3) transfer the physical force from the hydrogel to encapsulated cells to stimulate more rapid bone repair. The objectives of the present research are 1) to evaluate the effect of LIPUS-induced acoustic radiation force on rat marrow derived stem cells using three different acoustic radiation forces, 2) to evaluate the effect of radiation force on cells encapsulated in collagen hydrogels of varying viscoelasticities to determine the relationship between applied force and hydrogel viscosity on cell behavior, and 3) to use acoustic force applied to hydrogels that have been loaded with cells and implanted in rat calvarial defects. Implanted hydrogels containing cells will be loaded transdermally using LIPUS induced acoustic radiation force after implantation into calvarial defects and during the healing process. It is anticipated that the parameters defined in the in vitro studies will result in enhanced in vivo calvarial defect healing in hydrogels under acoustic radiation force when compared to either hydrogels alone or acoustic radiation force alone.